A Forward Iterator is an iterator that corresponds to the usual
intuitive notion of a linear sequence of values. It is possible to use
Forward Iterators (unlike Input Iterators and Output Iterators)
in multipass algorithms. Forward Iterators do not, however, allow
stepping backwards through a sequence, but only, as the
name suggests, forward.
A type that is a model of Forward Iterator
may be either mutable or immutable, as defined in the Trivial Iterators
requirements.
Forward Iterator does not define any new expressions beyond those
defined in Input Iterator. However, some of the restrictions
described in Input Iterator are relaxed.
Name
Expression
Type requirements
Return type
Preincrement
++i
X&
Postincrement
i++
X
Expression semantics
Forward Iterator does not define any new expressions beyond those
defined in Input Iterator. However, some of the restrictions
described in Input Iterator are relaxed.
Name
Expression
Precondition
Semantics
Postcondition
Preincrement
++i
i is dereferenceable
i points to the next value
i is dereferenceable or past-the-end. &i == &++i. If i == j, then
++i == ++j. [1]
[1]
The restrictions described in Input Iterator have been
removed. Incrementing a forward iterator does not invalidate copies
of the old value and it is guaranteed that, if i and j are
dereferenceable and i == j, then ++i == ++j. As a consequence
of these two facts, it is possible to pass through the same
Forward Iterator twice.